1. Field of the Invention
The present invention relates to a resin-coated seamless can, and more particularly to a resin-coated seamless can which exhibits excellent corrosion resistance and excellent shock resistance.
2. Description of the Related Art
Conventionally, as a side seamless can, there has been known a can which is produced by a following method. That is, a metal blank such as an aluminum plate, a tin plate or a tin-free steel plate is subjected to at least one drawing stage. The drawing stage is conducted between a drawing die and a punch to form a cylindrical cup comprising a barrel portion free of seams on a side surface thereof and a bottom portion integrally connected to the barrel portion which is also free of seams. Then, if desired, the barrel portion may be subjected to ironing between an ironing punch and a die to reduce the thickness of the barrel portion (drawing/ironing working).
Further, in place of such ironing, it is also known in the art to reduce the thickness of the side wall by performing thinning drawing forming which reduces the thickness of the side wall by bending and elongating the side wall at a curvature corner part of a redrawing die (drawing/thinning drawing forming).
Further, there has been also known thinning drawing forming and ironing working which reduces the thickness of the side wall portion by adding ironing working at the time of performing thinning drawing working (drawing/thinning drawing forming and ironing working).
Methods for coating an organic resin film onto the side seamless can include a method of applying an organic coating onto a formed can which is a common and widely used technique and, in addition, a method of laminating a resin film onto a metal blank before a can is formed. Further, in the production of a can by thinning drawing forming, the use of a metal sheet coated with polyester, vinyl organosol, epoxy, phenolic or acryl has been known in the art.
A considerable number of proposals have been made concerning a method for coating a thermoplastic resin film typically made of thermoplastic polyester onto a metal substrate. For example, there have been adopted a method which laminates a biaxially stretched film onto a metal substrate using heat-bonding directly or by way of a primer layer for adhesion (for example, Japanese Laid-open Patent Publication 101930/1991, Japanese Laid-open Patent Publication 4229/1993, Japanese Laid-open Patent Publication 172556/1994) and a method which extrudes molten resin and coats the molten resin onto a metal substrate (Japanese Laid-open Patent Publication 86308/1998).
With respect to the seamless can which is formed by drawing/ironing working, drawing/thinning drawing forming or drawing/thinning drawing forming and ironing working (hereinafter, these working methods being referred to as “drawing/thinning drawing forming and/or ironing working”), the demand for the enhancement of degree of working has been increasing year by year. Accordingly, it has been difficult for a resin-coated metal sheet for can use which has been conventionally adopted and is formed by laminating a biaxially stretched film on a metal sheet by heat adhesion to catch up with the harsh demand for working which becomes stricter year by year.
To the contrary, with respect to the resin-coated metal sheet for can use which is produced by extruding and coating molten resin on a metal substrate or by laminating a cast film to the metal substrate, since the resin is held in the non-oriented state, the tolerance for endurance against the above-mentioned harsh working is large. Due to such a reason, the resin-coated metal sheet for can use which is produced by extruding and coating molten resin on a metal substrate or by laminating a cast film to the metal substrate has been recently used.
The thermoplastic resin-coated metal sheet which is produced by extruding molten resin onto the metal substrate or by laminating the cast film to the metal substrate is formed into a cylindrical cup by drawing forming. Thereafter, a seamless can which has a large height and can reduce a thickness of a side wall thereof is formed by thinning drawing forming and/or ironing working. Such a seamless can largely differs between a can bottom portion and a can barrel portion with respect to the state of an adhesive interface formed between a coated resin layer and the metal substrate.
That is, with respect to the can bottom portion, since the degree of working is not strict, the adhesive interface substantially remains unchanged in a form of the original resin-coated metal sheet before working, while with respect to the can barrel portion, since the degree of working is strict, an internal stress at the adhesive interface derived from the working of the coated resin layer is increased, whereby there has been a case that the occurrence of peeling-off or cracks at the adhesive interface is recognized. Accordingly, irrespective of the presence or the absence of a surface treatment layer on a surface of the metal substrate, the corrosion resistance and the shock resistance when the resin-coated metal sheet is formed into a seamless can are degraded.
With respect to corrosion resistance of the seamless can formed of the thermoplastic resin-coated metal sheet, corrosion under a film which is referred to as an under film corrosion (UFC) in general is liable to progress. This corrosion is a phenomenon that although there seems to be no problem in terms of protection with respect to a film layer which covers a metal substrate in appearance, the metal substrate underneath the film layer is subjected to corrosion. The resin-coated metal sheet is requested to prevent such a corrosion.
As one of shock resistances, there exists dent resistance which is measured when the resin-coated metal sheet is formed into an actual canned product. This dent resistance is the property which the resin-coated metal sheet is required to have. That is, the dent resistance is the property which ensures the resin-coated metal sheet to completely maintain the adhesiveness of the film and the protection even when an indentation called a dent is formed in the canned product due to a fall of the canned product or a collision of canned products.
Accordingly, it is an object of the present invention to provide a resin-coated seamless can which is formed of a resin-coated metal sheet produced by coating thermoplastic resin on a surface of a metal substrate and exhibits the excellent corrosion resistance and the excellent shock resistance.